Perihelion Activity of (3200) Phaethon Is Not Dusty

We present an analysis of asteroid (3200) Phaethon using coronagraphic observations from 2008 to 2022 by the COR2 cameras onboard the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft. Although the asteroid cannot be confidently detected in individual images, we managed to spot it in image stacks combined from the same sets of perihelion observations, yet only when observed at low phase angles ($\lesssim$30\deg) but not at large phase angles ($\gtrsim$150\deg). The lack of a strong forward-scattering enhancement that is expected for dust grains having sizes comparable to transmitted wavelengths thereby implies that the perihelion activity of Phaethon is highly unlikely to be relevant to the ejection of dust grains as suggested by earlier studies based on STEREO's HI-1 observations. Assuming the observed activity of Phaethon is caused by dust ejection will lead to an insurmountable discrepancy in the inferred amount of dust no less than an order of magnitude between the HI-1 and COR2 observations. Rather, we speculate that the perihelion activity is caused by sodium and/or iron emissions, the former of which may have become transmittable due to an ageing effect of the HI-1 cameras. The modelled emission flux is qualitatively similar to the HI-1 observations in the case where the peak of the atomic production rate is delayed by $\sim$1 day from perihelion. We encourage future observations of Phaethon at small heliocentric distances to verify our conjecture.Comment: Revised according to referees' comment

New Active Asteroid (6478) Gault

Main-belt asteroid (6478) Gault was observed to show cometary features in early 2019. To investigate the cause, we conducted {\it BVR} observations at Xingming Observatory, China, from 2019 January to April. The two tails were formed around 2018 October 26--November 08, and 2018 December 29--2019 January 08, respectively, and consisted of dust grains of $\gtrsim$20 $\mu$m to 3 mm in radius ejected at a speed of $0.15 \pm 0.05$ m s$^{-1}$ and following a broken power-law size distribution bending at grain radius $\sim$70 $\mu$m (bulk density 1 g cm$^{-3}$ assumed). The total mass of dust within a $10^4$ km-radius aperture around Gault declined from $\sim$$9 \times 10^6$ kg since 2019 January at a rate of $2.28 \pm 0.07$ kg s$^{-1}$, but temporarily surged around 2019 March 25, because Earth thence crossed the orbital plane of Gault, within which the ejected dust was mainly distributed. No statistically significant colour or short-term lightcurve variation was seen. Nonetheless we argue that Gault is currently subjected to rotational instability. Using the available astrometry, we did not detect any nongravitational acceleration in the orbital motion of Gault.Comment: 6 pages, 4 figures, 2 table

Fragment Dynamics in Active Asteroid 331P/Gibbs

We present a dynamical analysis of the fragmented active asteroid 331P/Gibbs. Using archival images taken by the Hubble Space Telescope from 2015 to 2018, we measured the astrometry of the primary and the three brightest (presumably the largest) components. Conventional orbit determination revealed a high-degree of orbital similarity between the components. We then applied a fragmentation model to fit the astrometry, obtaining key parameters including the fragmentation epochs and separation velocities. Our best-fit models show that Fragment B separated from the primary body at a speed of $\sim$1 cm s$^{-1}$ between 2011 April and May, whereas two plausible scenarios were identified for Fragments A and C. The former split either from the primary or from Fragment B, in 2011 mid-June at a speed of $\sim$8 cm s$^{-1}$, and the latter split from Fragment B either in late 2011 or between late 2013 and early 2014, at a speed of $\sim$0.7-0.8 cm s$^{-1}$. The results are consistent with rotational disruption as the mechanism causing the cascading fragmentation of the asteroid, as suggested by the rapid rotation of the primary. The fragments constitute the youngest known asteroid cluster, providing us with a great opportunity to study asteroid fragmentation and formation of asteroid clusters.Comment: 13 pages, 5 figures, 4 tables. Accepted by A